I didn't notice it so much on WFLD but then the way the Bears played yesterday I may have just mistaken it for Bears fans groaning!
I did notice it on WBBM (also Chicago) earlier the day, though. I tried resetting my receiver and it didn't make any difference.
This seems to happen fairly regularly on athletic events - I suspect it might be related to the compression algorithms used and the fact that fast action sporting events don't compress nearly as well as other programming (I am no expert on compression so this is pure speculation).